In civil engineering and construction, materials such as soil, rocks, cement, mortar, concrete and metallic materials such as steel are mainly used. In particular, concrete and mortar having cement as their main component are extremely useful for producing various types of architecture and civil engineering constructions. They are inexpensive, have good workability and can easily be formed into various shapes. Concrete articles obtained show excellent mechanical strength, and have become a fundamental element of material civilization.
The concrete articles contemplated in the present invention are molded concrete products such as concrete pipes for water supply and sewerage, culverts, manholes, tunnel walls, gutters and various inlets, shaped surfaces of fresh concrete articles being able to be coated with resins during the molding process. Among such concrete articles, hume concrete pipes, for example, have such advantages as being inexpensive and strong; however, they fail to show good resistance to acids and chemicals. In a corrosive environment, the inner wall of the hume concrete pipe will corrode so that a fragile layer will be formed with the lapse of time.
In order to prevent such corrosion of the pipe, the inner and outer walls of the pipe are often coated with; (1) a coating composition containing a resin such as a polyester resin, a urethane resin, an epoxy resin or an acrylic resin; or (2) a film or sheet of polyvinyl chloride or polyolefin, a non-woven fabric or a reticulate product of polyester, polyolefin, etc. together with an adhesive resin (Japanese Patent Laid-Open Nos. 48-55553, 59-62794, 62-71614 and 63-275890).
When forming coats with the aforementioned materials, it is demanded that the coating compositions used should have strong adhesion properties to the pipe body, excel in water resistance and in chemical resistance. It is also demanded that the coat formed should have appropriate softness in order to perform follow-up occlusion when the concrete has cracked, and have strong resistance to abrasion by the violently running clean/waste water.
However, because of the difference in a coefficient of thermal expansion between the corrosion-resistant layer obtained from the coating composition and the concrete body, the coating layer tends to peel from the concrete body. Also, the layer obtained from the coating composition does not have sufficient strength and chemical resistance for a composite pipe. Furthermore, such corrosion-resistant layer has rigidity but lacks softness; therefore, it is not capable of performing follow-up occlusion when the concrete has cracked.
With respect to the adhesion of the coating composition to the concrete body, it is desired that the coating composition should be capable of forming a coat strongly adhered to the concrete body even when the surface of the concrete body is wet or covered with water. However, in the case of coating compositions containing commercially available epoxy resins such as bisphenol A-type epoxy resins or acrylic resins, these resins do not have good adhesion properties to the concrete, and when water exists on the surface of the concrete, it is extremely difficult to form a resin coat strongly adhered to the concrete. In addition, layers obtained from these coating compositions will suffer form poor gas barrier properties and poor solvent resistance.
Moreover, since these resin coats have rigidity but lack softness as explained earlier, they are not capable of performing follow-up occlusion when the concrete has cracked. In order to improve the crack follow-up ability, softening of resins can be considered, but softened resins would be poor in chemical resistance and water resistance. There also exists another problem that the abrasion resistance of layers obtained from softened resins is nowhere near the satisfactory level.
On the other hand, adhering sheets or non-woven fabrics of various resins to the inner wall of the pipe body is a very complicated work, and the adhesive agents used to adhere these sheets or non-woven fabrics to the pipe have the same problems as the aforementioned coating compositions. Therefore, the adhesion of these sheets or non-woven fabrics will not be satisfactory, and consequently show poor gas barrier properties.
Thus, the conventional methods of bestowing corrosion resistance to the outer and inner walls of the concrete pipes are not satisfactory, and their execution of coating work is quite complicated.
Accordingly, an object of the present invention is to provide a corrosion-resistant, durable concrete article comprising a layer of a cured epoxy resin modified with a polysulfide, which layer is well adhered to a concrete body, and to a method of producing such concrete articles.